In recent years, an inkjet recording method has been often used in the field of production of large-sized advertisement requiring a large image area for outdoor use. Substrates to be used for such advertisements are tough polyvinyl chloride sheets durable for outdoor use such as a sheet made of a vinyl chloride resin itself or a tarpaulin sheet made of a composite material. As an ink for inkjet printing to be printed on such a polyvinyl chloride sheet, for example, a photocurable ink composition for inkjet printing free of volatile components has been suggested (see, for example, Patent Literatures 1 and 2).
The ink compositions for inkjet printing according to Patent Literatures 1 and 2 are cured using a high-energy light source, such as a metal halide lamp, which has been conventionally used as a light source. The use of such a high-energy light source, such as a metal halide lamp, however, raises problems that ozone is generated, a large-sized irradiation device is, needed, and the lamp life is short.
For this reason, a low-energy light-emitting diode lamp (LED lamp) has been recently used as a light source.
Therefore, a photocurable ink composition for inkjet printing that is sufficiently cured, has good printability (e.g. adhesion, improvement in cockling) and safety (e.g. flash point, skin irritation) even when the composition is cured by low energy has been required. A photocurable ink composition for inkjet printing capable of meeting such requirements has been suggested.
Examples of such a photocurable ink for inkjet printing to be used include: (1) a photocurable ink composition for inkjet printing having a flash point of 70° C. or higher which contains a pigment, benzyl acrylate, N-vinylcaprolactam, an acrylated amine compound containing two photopolymerizable functional groups and two amino groups in a molecule, a photopolymerization initiator, and a sensitizer (see Patent Literature 3); (2) a photocurable ink composition for inkjet printing which contains a pigment, a compound containing a (meth)acrylate group and a vinyl ether group in a molecule, a (meth)acrylated amine compound, a hindered amine compound other than (meth)acrylated amine compounds, and a photopolymerization initiator (see Patent Literature 4); and (3) a photocurable ink composition for inkjet printing which contains a pigment, a compound containing a specific amount of a (meth)acrylate group and a vinyl ether group in a molecule, and a specific amount of N-vinylcaprolactam (at least one of phenoxy acrylate and polyfunctional acrylate is used in combination therewith in all examples, see Patent Literature 5).
An energy-saving has ranged to inkjet printers, and an inkjet head capable of being driven at lower energy has been developed. Further, a method of allowing high speed printing by increasing a driving frequency of an inkjet head has been developed, and further an inkjet head capable of ejecting a large number of fine liquid droplets has been developed for achieving high definition recording images without granular feeling.
In order to realize such ideas, the viscosity of the ink composition needs to be low. Because, in the inkjet recording method in which an ink composition is filled in a nozzle and is ejected from the nozzle by an inner pressure of a head, the time for filling the nozzle with the ink composition is directly related to a printing speed, and the inner pressure necessary to eject the ink composition is directly related to a driving energy.
The time for filling the nozzle with an ink composition and the inner pressure necessary to eject an ink composition are greatly controlled by the viscosity of the ink composition. That is, the time for filling the nozzle is small in case of a low-viscosity ink composition, and the inner pressure necessary to eject an ink composition can be suppressed to be low. Further, finer nozzles need longer filling time, and therefore low viscosity is a significantly important property for the ink composition.
However, the photocurable ink compositions for inkjet printing according to Patent Literatures 3 to 5 are relatively highly viscous, and do not meet the above ideas.
If a method of warming and heating an ink composition is used, the viscosity of the ink composition can be reduced while maintaining the ink performance, but the warming and heating need additional energy, and cause damage to an inkjet head.